Integrated Grating Coupler System

ABSTRACT

A grating coupler having first and second ends for coupling a light beam to a waveguide of a chip includes a substrate configured to receive the light beam from the first end and transmit the light beam through the second end, the substrate having a first refractive index n1, a grating structure having curved grating lines arranged on the substrate, the grating structure having a second refractive index n1, wherein the curved grating lines have line width w and height d and are arranged by a pitch Λ, wherein the second refractive index n2 is less than first refractive index n1, and a cladding layer configured to cover the grating structure, wherein the cladding layer has a third refractive index n3.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional application of U.S. patent applicationSer. No. 16/260,747, filed Jan. 29, 2019, and claims priority to theSer. No. 16/260,747 application claiming priority to a provisionalapplication Ser. No. 62/773,721 filed Nov. 30, 2018, which is herebyincorporated by reference.

FIELD OF THE INVENTION

This invention generally relates to grating couplers for optical chips,also called photonic integrated circuits (PICs), and more particularlyto grating coupler system connecting one active chip and one passiveoptical chip.

BACKGROUND OF THE INVENTION

The target application is a hybrid integration of an active optical chipsuch as one containing InP waveguides and a passive chip such as onecontaining silicon and/or silicon nitride waveguides. The expectedproperties are, large tolerance to mis-alignment, easiness of bondingprocesses, and high coupling efficiency.

Silicon photonics offer many advantages of which the fabrication cost isthe most important factor. Furthermore, high refractive index contrastbetween the silicon waveguide and the surrounding silicon dioxide layersoffer tight bending with low loss possible, leading to higher densityand complexity PICs. Silicon nitride waveguides offer similar low costcapabilities, with lower optical loss property. On the other hand, thereis no reliable optical gain or emission capability with direct currentinjection. Therefore, hybrid integtation of active PICs (such as InP,GaAs, or GaN-based ones) with passive silicon photonics PICs become veryimportant to achieve low cost, full functionality, and high densityPICs.

However, optically connecting two waveguides precise requires precisealignment typically with sub-micron accuracy, due to narrow waveguidesand thus fast diverging beam on both sides. There is a need to connecttwo optical chips with larger tolerance with high coupling efficiency.

SUMMARY OF THE INVENTION

Some embodiments of the present disclosure are based on recognition thatlong period grating on a passive waveguide from an optical chip createsshallow angle emission towards the substrate side, diffracted at thechip facet (second end) at a steeper angle, and the coupled to thepassive optical chip through a grating coupler.

In accordance to some embodiments, a novel grating coupler system isrealized by a grating coupler having first and second ends for couplinga light beam to a waveguide of a chip including a substrate configuredto receive the light beam from the first end and transmit the light beamthrough the second end, the substrate having a first refractive indexn1; a grating structure having grating lines arranged on the substrate,the grating structure having a second refractive index n2, wherein thegrating lines have line width w and height d and are arranged by a pitchΛ, wherein the second refractive index n2 is greater than firstrefractive index n1; and a cladding layer configured to cover thegrating structure, wherein the cladding layer has a third refractiveindex n3, wherein the third refractive index n3 is less than the secondrefractive index n2, wherein the cladding layer is arranged so as toreflect the light beam diffracted from the grating structure towardbelow the cladding layer.

Further, another embodiment of the present invention is based onrecognition that an integrated grating coupler system includes a gratingcoupler formed on a first chip, the grating coupler having first andsecond ends for coupling a light beam to a waveguide of a second chip,wherein the grating coupler comprises a substrate configured to receivethe light beam from the first end and transmit the light beam throughthe second end, the substrate having a first refractive index n1; agrating structure having grating lines arranged on the substrate, thegrating structure having a second refractive index n2, wherein thegrating lines have line width w and height d and are arranged by a pitchΛ, wherein the second refractive index n2 is greater than firstrefractive index n1; and a cladding layer configured to cover thegrating structure, wherein the cladding layer has a third refractiveindex n3, wherein the third refractive index n3 is less than the secondrefractive index n2; and a gain region connected to the first end of thegrating coupler either directly or through a waveguide, wherein thelaser structure comprises a substrate identical to a substrate of thegrating coupler; an active layer having a first thickness d1, the activelayer being arranged on the substrate, wherein the active layer isconnected to the grating structure of the grating coupler; and acladding layer arranged on the active layer, the cladding layer having asecond thickness d2 is connected to the cladding layer of the gratingcoupler.

Yet further, according to another embodiment of the present invention,an integrated grating coupler system includes a grating coupler formedon a first chip, the grating coupler having first and second ends forcoupling a light beam to a waveguide of a second chip, wherein thegrating coupler comprises: a substrate configured to receive the lightbeam from the first end and transmit the light beam through the secondend, the substrate having a first refractive index n1; a gratingstructure having grating lines arranged on the substrate, the gratingstructure having a second refractive index n2, wherein the grating lineshave first line width w1 and first height d1 and are arranged by a firstpitch Λ1, wherein the second refractive index n2 is greater than firstrefractive index n1; a cladding layer to cover the grating structure,wherein the cladding layer has a third refractive index n3, wherein thethird refractive index n3 is less than the second refractive index n2;and a laser structure connected to the first end of the grating coupler,wherein the laser structure comprises a substrate identical to asubstrate of the grating coupler; an active layer to emit the light beamto the grating coupler, the active layer having a first thickness d1 anda fifth refractive index n4, the active layer being arranged on thesecond substrate, wherein the active layer is connected to the gratingstructure of the grating coupler; a laser-cladding layer arranged on theactive layer, the laser-cladding layer having a second thickness d2 anda sixth refractive index n5 is connected to the cladding layer of thegrating coupler; and first and second electrodes to apply a voltagethrough the laser structure; and a waveguide device to mount the gratingcoupler and the laser structure, wherein the waveguide device comprisesa first cladding layer connected to bottoms of the grating coupler andthe laser structure the first cladding layer having a seventh refractiveindex n7; a waveguide structure having a second grating structure,wherein the second grating structure includes second grating linesarranged in part of the waveguide structure to couple a laser beam fromthe grating coupler to the waveguide structure, the waveguide structurehaving a eighth refractive index n8, wherein the grating lines havesecond line width w2 and second height d2 and are arranged by a secondpitch Λ2, wherein the eighth refractive index n8 is greater than seventhrefractive index n7; and a waveguide substrate connected to thewaveguide structure, the waveguide substrate having a ninth refractiveindex n9, wherein the ninth refractive index n9 is less than the eighthrefractive index n8.

BRIEF DESCRIPTION OF THE DRAWINGS

The presently disclosed embodiments will be further explained withreference to the attached drawings. The drawings shown are notnecessarily to scale, with emphasis instead generally being placed uponillustrating the principles of the presently disclosed embodiments.

FIG. 1 shows a cross-sectional view of the integrated grating couplersystem according to the invention;

FIG. 2A show the definition of two angles θ and θ_(a);

FIG. 2B shows the calculated relationship between the waveguideeffective refractive index and the diffracted light angles;

FIG. 3A shows the simulated coupling efficiency between the two opticalchips as a function of wavelength;

FIG. 3B shows the simulated coupling efficiency with different silicongrating length (positions);

FIG. 4A shows this invention containing sub-gratings;

FIG. 4B shows the conventional gratings without sub-gratings;

FIG. 4C shows the simulated coupling efficiency with and withoutsub-gratings;

FIG. 4D shows a grating shape wherein both edges are slanted;

FIG. 4E shows a grating shape wherein a slanted edge is on one side;

FIG. 4F shows a grating with sub-gratings on one side;

FIG. 4G shows a grating with sub-gratings with varied width and/orspacing;

FIG. 5A shows a cross-sectional view of a grating etched directly ontothe waveguide layer, with an optional dielectric film on the claddinglayer;

FIG. 5B shows a cross-sectional view of a grating above the waveguidelayer;

FIG. 5C shows a cross-sectional view of a grating written directly onthe waveguide layer and a guide (separate confinement) layer coveringthe grating;

FIG. 5D shows a cross-sectional view of a grating made of separatedsegments of a waveguide layer;

FIG. 5E shows a cross-sectional view of a grating made of separatedsegments of a waveguide layer embedded in a separate confinement layer;

FIG. 6 shows a cross-sectional view of a grating with multiple layers ofanti-reflection dielectric films on the facet;

FIG. 7 shows the simulated facet reflectivity as a function of incidentangle, wherein no coating, one layer coating (Si₃N₄), and two layercoating (Si₃N₄ and SiO₂ films) are applied;

FIG. 8A show the cross-section view of the grating coupler system,wherein the grating lines are curved;

FIG. 8B shows the top view of the grating coupler system of FIG. 8A,wherein the grating lines are curved. θ₁ and θ₂ are the angles for theconcentric grating lines for the first and second chips, respectively;

FIG. 9A show the cross-section view of the grating coupler system, wherelinear gratings are connected to the waveguides through taperedwaveguides;

FIG. 9B shows the top view of the grating coupler system of FIG. 9A;

FIG. 10A shows a cross-sectional view of a grating coupler system,wherein a gain section, or a laser section (DFB or DBR laser) isattached to the first end;

FIG. 10B shows a top view of a grating coupler system of FIG. 10;

FIG. 11A shows a cross-sectional view of a grating coupler systemconfigured as a tunable laser with an optical chip with a gratingcoupler integrated with an active section and wavelength selectivereflector, attached on a second optical chip with a grating coupler anda wavelength selective reflector; and

FIG. 11B shows a top view of a grating coupler system of FIG. 11A.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The following description provides exemplary embodiments only, and isnot intended to limit the scope, applicability, or configuration of thedisclosure. Rather, the following description of the exemplaryembodiments will provide those skilled in the art with an enablingdescription for implementing one or more exemplary embodiments.Contemplated are various changes that may be made in the function andarrangement of elements without departing from the spirit and scope ofthe subject matter disclosed as set forth in the appended claims.

Specific details are given in the following description to provide athorough understanding of the embodiments. However, understood by one ofordinary skill in the art can be that the embodiments may be practicedwithout these specific details. For example, systems, processes, andother elements in the subject matter disclosed may be shown ascomponents in block diagram form in order not to obscure the embodimentsin unnecessary detail. In other instances, well-known processes,structures, and techniques may be shown without unnecessary detail inorder to avoid obscuring the embodiments. Further, like referencenumbers and designations in the various drawings indicated likeelements.

Furthermore, embodiments of the subject matter disclosed may beimplemented, by use of at least in part, or combinations of parts of thestructures described below.

Overview of Embodiments of the Present Disclosure

Optical coupling between two optical chips constitute the most importantpart of hybrid PICs. The easiness of alignment and high couplingefficiency are very important factors. Grating couplers offer thesecapabilities.

There are multiple factors in achieving high coupling efficiency forthis configuration.

FIG. 1 shows a cross-sectional view of the integrated grating couplersystem 100 according to the invention. The first optical chip (firstchip) 105 is made on an InP substrated 110, containing an InGaAsPwaveguide layer 130, InP cladding layer 120, and a first grating 140.The second optical chip (second chip) 145 comprises of a siliconsubstrate 150, a buried SiO₂ layer (also called a BOX layer) 160, asilicon (Si) waveguide layer (also called silicon-on-insulator, or SOI)170, and a SiO₂ cladding layer 180, and the second grating 190 etchedonto the silicon waveguide layer. The diffracted light in the firstoptical chip 105 propagates through the InP substrate 110 and the firstoptical chip facet 195, and is coupled into the grating 190 on thesecond optical chip 145.

Here, the grating pitch Λ is the distance between the rising edges ofthe grating, w is the line width of the main tooth, and d is thethickness of the grating. In the first optical chip, grating diffractslight towards the substrate as a shallow angle, which is furtherdiffracted at the chip fact to a steeper angle. The light is focused onthe grating in the second chip and is guided to its waveguide. Theoperating wavelength of 1530-1570 nm, the typical grating pitch Λ is5-15 μm, and the typical grating line width w is 10-60% of the gratingpitch, depending on whether sub-gratings are included, or how thesub-gratings are designed. The typical grating thickness d is 0.2-1 μm.

FIG. 2A show the definition of θ and θ_(a), wherein θ is the anglebetween the InGaAsP waveguide layer 210 and the diffracted light 220,and θ_(a) is the angle between the facet normal 230 and the diffractedlight 240 passing through the facet 250.

FIG. 2B shows the relationship between the waveguide effectiverefractive index and the diffracted light angle θ and θ_(a), before andafter facet diffraction, respectively.

The relationship between the diffraction angle, refractive indices, andthe grating pitch can be expressed as,

${{k_{0}n_{eff}} - {n_{s}\cos \; \theta}} = {m\frac{2\; \pi}{\Lambda}}$

where A is the pitch of the grating, k₀=2π/λ is the wave vector invacuum, λ is the wavelength in vacuum, n_(eff) is the effectiverefractive index of the waveguide, n_(s) is the refractive index of thesubstrate, θ is the angle between the propagation direction within thewaveguide and the diffraction angle as defined in FIG. 2A, and m is theorder of diffraction. From practical point of view, first-order gratingusually offers the highest coupling efficiency, so m=1 is used in thisinvention.

The angle between the diffracted light into the air from the facet andthe original waveguide propagation angle (facet normal) θ_(a) as definedin FIG. 2B can be expressed as,

n_(s) sin θ=sin θ_(a)

The diffraction angle from the grating in the first optical chip has tobe in a certain range, typically 10° and 20° within the optical chip. Ifit is below 10°, the diffraction light from the chip facet is alsoshallow (less than ˜33°), and coupling to a second optical chip makesless efficient. On the other hand, if the emission angle is more than20°, most of the light is reflected at the chip facet, making thecoupling very inefficient. Therefore, it is important to keep thediffraction angle between 10 and 20 degrees. Assuming n_(s)=3.169 for atypical InP at the wavelength of 1550 nm, n_(eff)=3.244, and Λ=8.5 μm,we obtain θ=15.0° and θ_(a)=55.1°.

FIG. 2B shows a plot of diffracted angles before and after the facetfrom the original propagation direction (i.e., facet normal), with twograting pitches, i.e., Λ=8.5 μm and Λ=10.5 μm are used. Depending on thepitch of the grating, there is an optimal waveguide effective refractiveindex. In other words, the grating pitch typically needs to be between 5μm and 15 μm.

FIG. 3A shows the simulated coupling coefficient as a function ofwavelength. The structural parameters are optimized such that thecoupling coefficient for the wavelength range of 1530-1565 nm ismaximized. The simulated coupling coefficient is 58.0% (−2.4 dB) at thepeak, and >49% for the wavelength range of 1530-1565 nm.

The optimized structure and the parameters are as follows. The InP sidestructure consists of 80 μm-thick InP substrate, 0.5 μm-thick InGaAsP(bandgap: 1.30 μm), 0.3 μm-deep etched grating, 0.47 μm-thick InP uppercladding layer, and 20 nm-thick SiO2 passivation layer. The length ofthe InP grating section is 200 μm. In order to achieve shallowdiffraction angle (15.0° within InP and 55.1° in the air) the initialpitch of the main grating in the left hand size is 8.5 μm. Further, inorder to achieve focusing effect, the grating pitch is varied as afunction of distance (linear pitch reduction: 0.1 μm per period). Thegrating has main teeth (width is 28% of the pitch) and 3 periods ofsub-gratings with 210 nm width and 220 nm spacing on each side of themain teeth. Sub-gratings are gratings whose period is smaller than themain grating. The grating is also apodized in that the width of theetched part is linearly shrunk for each period. The facet has a pair ofSiO₂ and Si₃N₄ coating. In the silicon side, we used two pairs (Si/SiO₂)of distributed Bragg reflectors (DBR) under the silicon waveguide. The0.22 μm-thick silicon waveguide is sandwiched between a 2.1 μm-thickSiO₂ lower cladding layer and a 1.04 μm-thick SiO2 upper cladding layer.The silicon grating depth is 87.5 nm. The length of the silicon gratingsection, starting from the end of the InP chip, is 32 μm. The silicongrating is apodized but does not have chirp.

FIG. 3B shows the simulated coupling efficiency when the silicon gratingsection length (i.e., the endpoint measured from the edge of the InPchip facet). When the wavelength is at 1560 nm, the coupling efficiencychanged within ±5% when the length changed by ±1.5 μm (29-32 μm). Thisis in contrast to typical chip-to-chip direct coupling, where submicronalignment accuracy is required.

Grating diffracts light in multiple direction corresponding to theFourier component of the diffraction grating. For example, third andfifth order diffraction, which do not contribute to the couplingefficiency, are related to the third and fifth order component of thegrating. Therefore, it is important to reduce those components byeffectively smoothing the grating. Sub-gratings, whose line width w₂ andpitch Λ₂ are much smaller than w and Λ, respectively, can be added on atleast one side of the main (primary) grating (FIG. 4A). The typicalw_(eff)/Λ is 0.4-0.6. the typical sub-grating line width w₂ is 0.02-0.3μm, and typical sub-grating pitch Λ₂ is 0.04-0.6 μm,

Also, the second Fourier component needs to be minimized. The duty cycle(w/Λ), needs to be close to 0.5 in the case of a primary grating withoutsub-gratings (FIG. 4B). In the case of FIG. 4A, the effective dutycycle, w_(eff)/Λ, where w_(eff) is the effective line width of thegrating, i.e., adding w of the main grating and cumulative w₂ from allthe subgratings.

FIG. 4C shows the simulated coupling efficiency with and withoutsub-gratings, wherein the duty cycle is optimized in each case. It isclearly shown that sub-gratings show much higher coupling efficiency dueto reduced diffraction in unnecessary directions.

The grating geometry does not necessarily have to be rectangular. Thirdand fifth-order diffraction can also be suppressed by incorporatinggratings with slanted edges, such as shown in FIG. 4D.

Suppressing the upward diffraction also increases downward diffractionwhich more efficiently couples into the grating on the second chip. Thiscan be achieved by slanting one of the edge of the gratings as shown inFIG. 4E, incorporating sub-gratings on one side of the grating (FIG.4F), or asymmetric sub-grating on each side of the grating (FIG. 4G).

FIG. 5A shows an example of the grating coupler of the first opticalchip 540, wherein the grating 515 is etched into the InGaAsP waveguidelayer 515 in the second refractive index, and is surrounded by an InPsubstrate 500 and an InP cladding layer 505 both of the first refractiveindex. This first optical chip can optionally have a SiO₂ or Si₃N₄dielectric layer 520 on top of an InP cladding layer 505. The grating515 typically diffracts light in both upward and downward directionswith similar intensity. Because of the shallow emission angle (10°-20°)in the first optical chip 540, the upward diffracted light 535 is veryefficiently reflected either at the cladding layer—dieletric layerinterface 525, or at the dielectric—air interface 530, and it isimportant not to disturb the surface. Therefore, flat interfaces 525 and530 are desirable and thickness of the cladding layer 505 and thedielectric layer 520 needs to be chosen such that the reflected beam iscombined with the originally downward diffracted light with the same orvery similar phase. Metal surface absorbs light and is not desirable.

A grating does not have to be directly etched into the waveguide layer.FIG. 5B shows an example wherein another InGaAsP layer of the thirdrefractive index 545, above the waveguide layer 510 forms a grating. Inthis case, the grating height is determined by the thickness of thegrating layer and not the etching depth, thus improving the processrobustness.

FIG. 5C shows an example wherein an InGaAsP separate confinement layer550 of the third refractive index covers the grating etched into thewaveguide layer. The refractive index difference between the separateconfinement layer 550 and the InGaAsP waveguide layer can be smallerthan the case depicted in FIG. 5A, thus improving the process robustnessdue to less sensitivity to the etch depth.

FIG. 5D shows an example wherein the InP substrate 500 and the InPcladding layer 505 of the first refractive index surrounds the InGaAsPwaveguide layer 510 of the second refractive index, which is fullyetched into grating segments 555. Since this creates unguided lightbetween the grating segments 555, and mismatch to the guided mode withinthe grating segment 555 is large, thus creating a very strong gratingand is adequate for short length grating region.

FIG. 5E shows a case wherein an InP substrate 500 and an InP claddinglayer 505 of the first refractive index surrounds an InGaAsP separateconfinement layer 550 of the third refractive index, wherein an InGaAsPwaveguide layer 510 with the second refractive index is fully etched andis buried. In this case, the optical confinement is mainly determined bythe separate confinement layer and the grating strength is determined bythe thickness of the waveguide layer, so the emission angle and couplingefficiency is robust to etching process variation.

FIG. 6 shows the schematic of a first optical chip 600, comprising of achip facet 610, the first dielectric film 620, and the second dielectricfilm 630. The optical beam impinging on the chip facet 610 is not normalto the surface, but is 10° to 20° described above. Therefore, dielectricfilms 610 and 620 have to be optimized for the incident angle, such astwo layers or more.

FIG. 7 shows the calculated transmittance of TE mode input as a functionof incident angle, wherein there is one layer (Si₃N₄) coating, two layer(Si₃N₄ and SiO₂), or no coating. The thicknesses of Si₃N₄ (firstdielectric film) and SiO₂ (second dielectric film) are optimized for 15°incident angle. One layer coating is effective in suppressingreflectivity at a certain angle, while two layer coating is effective inlowering reflectivity in a broad angle range. It is shown thatreflectivity increases significantly above 16° incident angle. The exactbehavior depends on the coating design, however, it is best to keep theincident angle below ˜20°.

The emitted beam diverges in the horizontal direction along thepropagation despite of relatively long gratings, typically 50 μm orlonger. It is therefore important to use chirped grating, i.e.,gradually changing the pitch Λ of the grating along the propagationdirection, thereby focusing the beam onto a narrow region of the gratingon the second chip.

In order to make the emitted beam closer to the beam shape (such asGaussian beam) more suitable for coupling into the second grating, thegrating strength can be altered along the propagation direction, alsocalled apodized grating.

The emitted beam also diverges in the lateral direction, that is, in thehorizontal direction normal to the propagation direction.

One way to narrow the lateral beam divergence is to use curved gratings,such as elliptic grating. FIGS. 8A and 8B show a cross-sectional viewand a top of the grating coupler system 800, respectively, wherein thefirst optical chip 810 and second optical chip 830 have ellipticalgratings 820 and 840, respectively. In one example, an InP waveguide 815with around 1 μm width is connected to the elliptic grating 820 with atleast 10° in full width. A silicon waveguide 835 with 0.5 μm width isalso connected to an elliptic silicon grating 840.

The most common way is to define elliptic curves is to follow theequation

qλ ₀ =n _(eff)√{square root over (y ² +z ²)}−zn _(t) cos θ_(c).

Here, z is the coordinate in the propagating direction, y is that in thelateral direction, q is an integer number for each grating line, θ isthe angle between the outgoing/incoming light and the chip surface,n_(t) is the refractive index of the environment, λ₀ is the vacuumwavelength, and n_(eff) is the effective index felt by the wave in thewaveguide with the grating. In the center (y=0), the grating lines aredetermined by the previously described methods whose cross-sections aredescribed in FIGS. 4 and 5.

Alternatively, we can use tilted grating so that the reflected lightwill not couple back into the original waveguide.

The above equation is derived to generate a circular beam. In thisgrating coupler system 800, due to the diffraction at the facet, acircular beam becomes an elliptical beam. Therefore, furthermodification to the grating lines 820 may be desirable to form a nearcircular beam observed at the grating 840 at the second optical chip.

Another way of narrowing the lateral beam divergence is to use taperedwaveguide. FIGS. 9A and 9B show a cross-sectional view and a top of thegrating coupler system 900, respectively, wherein the first optical chip910 and second optical chip 930 have elliptical gratings 920 and 940,respectively. In one example, an InP waveguide 915 with ˜1 μm width istapered to the width to 5-20 μm, and then it is connected to a linearInP grating 920, whose cross-section is defined in FIGS. 4 and 5. Thelength of the taper section can be 5-100 μm. The grating on the secondchip 930 can be a linear width grating 940. Alternatively, it can be anelliptical grating 840 such as shown in FIG. 8B.

It is also important to increase the coupling efficiency for the gratingin the second chip, typically based on silicon substrate, and siliconwaveguide is sandwiched between two SiO₂ layers. There are many ways toincrease it.

Bragg reflector (a pair of layers with different refractive indices)between the silicon substrate and the bottom of the SiO₂ layer.

-   Subgrating on at least one side of the main grating teeth.-   Silicon amorphous layer above the upper SiO₂ cladding layer.-   Metallic layer between the silicon substrate and the bottom of the    SiO₂ layer.

Alternatively, a Si₃N₄ waveguide, sandwiched between two SiO2 layers,can also be used for guiding the incoming light.

Also, a SiO2 substrate can be used for the second optical chip

In one embodiment of the invention, the grating coupler system 1000 cancomprise of a first optical chip with a waveguide 1020 and a grating1050 connected to a gain region 1035 comprising of an active layer 1030such as a multi quantum well (MQW) structure (FIGS. 10A and 10B). MQWstructure is a very efficient gain medium. The light generated oramplified in the gain region 1035 through a current injection providedby the electrode 1040 can be guided to the grating coupler 1050 to becoupled to the second optical chip 1060. The gain region can also bepart of a DFB or a DBR laser, wherein the laser light is generated. Inthis case, a substrate of DFB or a DBR laser may be identical to thesubstrate of the grating coupler system 1000.

FIGS. 11A and 11B show a cross-sectional view and a top of a tunablelaser based on an integrated grating coupler system 1100, respectively,wherein the first optical chip 1110 and second optical chip 1150 formthe laser cavity. In this case, a wavelength selective reflector 1115,whose refractive index is controlled by temperature or injected carrierthrough the first electrode 1120, an active section 1125 comprising ofan MQW structure with current injected through the second electrode1130, a grating coupler 1135, and a third electrode 140 are on a firstoptical chip 1110, formed on a substrate made of materials such as InP,GaAs, or GaN. A second optical chip 1150 comprises of a siliconsubstrate, BOX layer 1155, silicon and/or silicon nitride waveguide1160, a grating coupler 1165, and a wavelength selective reflector 1170whose refractive index can be controlled by temperature or injectedcarrier through a second pair electrodes 1175 and 1185. A phasecontroller can be on either chip. In FIG. 11B, a ring resonator isdepicted as an example of a wavelength selective mirror, however, it canalso be a sampled grating distributed Bragg reflector. These wavelengthselectable mirrors have multiple reflection peaks, wherein the peakwavelengths can be tuned by the injected current, applied reverse biasvoltage, or temperature controlled by a heater.

According to aspects of integrated grating coupler system describedabove, with respect to the integrated grating coupler system, a ratiow/Λ between the line width w and the pitch Λ over can be arranged to beapproximately 0.5 for reducing a second order diffraction of the lightbeam from the grating structure.

Further, a dielectric film may be arranged on the cladding layer, andthe substrate of the first chip may be an InP substrate. Also in somecases, the substrate of the second chip can be a Si substrate, thesecond chip may include a Si waveguide, and the first wavelengthselectable reflector may include a sampled grating Bragg reflector.

Although the present disclosure has been described with reference tocertain preferred embodiments, it is to be understood that various otheradaptations and modifications can be made within the spirit and scope ofthe present disclosure. Therefore, it is the aspect of the append claimsto cover all such variations and modifications as come within the truespirit and scope of the present disclosure.

What is claimed is:
 1. An integrated grating coupler system comprising:a grating coupler formed on a first chip, the grating coupler havingfirst and second ends for coupling a light beam to a waveguide of asecond chip, wherein the grating coupler comprises: a substrateconfigured to receive the light beam from the first end and transmit thelight beam through the second end, the substrate having a firstrefractive index n1; a grating structure having grating lines arrangedon the substrate, the grating structure having a second refractive indexn2, wherein the grating lines have first line width w1 and first heightd1 and are arranged by a first pitch Λ1, wherein the second refractiveindex n2 is greater than first refractive index n1; a cladding layer tocover the grating structure, wherein the cladding layer has a thirdrefractive index n3, wherein the third refractive index n3 is less thanthe second refractive index n2; and a laser structure connected to thefirst end of the grating coupler, wherein the laser structure comprises:a substrate identical to a substrate of the grating coupler; an activelayer to emit the light beam to the grating coupler, the active layerhaving a first thickness d1 and a fifth refractive index n4, the activelayer being arranged on the second substrate, wherein the active layeris connected to the grating structure of the grating coupler; alaser-cladding layer arranged on the active layer, the laser-claddinglayer having a second thickness d2 and a sixth refractive index n5 isconnected to the cladding layer of the grating coupler; and first andsecond electrodes to apply a voltage through the laser structure; and awaveguide device to mount the grating coupler and the laser structure,wherein the waveguide device comprises: a first cladding layer connectedto bottoms of the grating coupler and the laser structure the firstcladding layer having a seventh refractive index n7; a waveguidestructure having a second grating structure, wherein the second gratingstructure includes second grating lines arranged in part of thewaveguide structure to couple a laser beam from the grating coupler tothe waveguide structure, the waveguide structure having a eighthrefractive index n8, wherein the grating lines have second line width w2and second height d2 and are arranged by a second pitch Λ2, wherein theeighth refractive index n8 is greater than seventh refractive index n7;and a waveguide substrate connected to the waveguide structure, thewaveguide substrate having a ninth refractive index n9, wherein theninth refractive index n9 is less than the eighth refractive index n8.2. The integrated grating coupler system of claim 1, wherein the laserstructure further comprises a first wavelength selective reflector and athird electrode to extract part of the light beam having a selectablewavelength, wherein the waveguide device comprises a second wavelengthselective reflector and second-pair electrodes to extract the part ofthe light beam having the predetermined wavelength.
 3. The integratedgrating coupler system of claim 1, wherein the laser structure furthercomprises first and second electrodes, wherein the first electrode iselectrically connected to the laser-cladding layer, wherein the secondelectrode is electrically connected to a surface of the first chip or asurface of the second chip.
 4. The integrated grating coupler system ofclaim 1, wherein the laser structure further comprises a phase controlregion, between the first and second wavelength selectable reflector,wherein the optical phase is controlled by an injected current, appliedreverse bias voltage, or a temperature controlled by a heater.
 5. Theintegrated grating coupler system of claim 1, wherein the gratingstructure further includes a waveguide layer to form a grating geometryconnecting the grating lines on the waveguide layer, wherein thewaveguide layer having a thickness d is arranged on the substrate. 6.The integrated grating coupler system of claim 5, wherein the gratingstructure further includes sub-gratings having line width w2 and theheight d and a second pitch Λ2, wherein the pitch Λ is greater than thesecond pitch Λ2, wherein the sub-gratings are arranged on at least oneside of each of the grating lines.
 7. The integrated grating couplersystem of claim 1, wherein the pitch Λ is changed from the first end tothe second end to focus the light beam on the waveguide of the chipaccording to a function of distances.
 8. The integrated grating couplersystem of claim 1, wherein a ratio w/Λ between the line width w and thepitch Λ over is arranged to be approximately 0.5 for reducing a secondorder diffraction of the light beam from the grating structure.
 9. Theintegrated grating coupler system of claim 1, further comprising an endanti-reflection film arranged on the second end.
 10. The integratedgrating coupler system of claim 9, wherein the end anti-reflection filmconsists of at least two layers with different materials.
 11. Theintegrated grating coupler system of claim 1, wherein the gratingcoupler includes a dielectric film arranged on the cladding layer. 12.The integrated grating coupler system of claim 1, wherein the thirdrefractive index n3 of the cladding layer is approximately the same asthe first refractive index n1 of the substrate.
 13. The integratedgrating coupler system of claim 1, wherein the grating coupler includesa second cladding layer arranged between the grating structure and thecladding layer, wherein the second cladding layer has a fourthrefractive index n4, wherein the fourth refractive index n4 is less thanthe third refractive index n3.
 14. The integrated grating coupler systemof claim 1, wherein the grating lines are elliptic grating linesarranged to focus the light beam to the waveguide of the chip.
 15. Theintegrated grating coupler system of claim 1, wherein the gratingcoupler includes a second waveguide layer arranged on the substrate,wherein the grating lines of the grating structure are separatelyarranged above the second waveguide layer and burred in the claddinglayer.
 16. The integrated grating coupler system of claim 1, wherein thegrating lines of the grating structure are separately arranged betweenthe substrate and the cladding layer.
 17. The integrated grating couplersystem of claim 1, wherein the substrate is an InP substrate.